device-raspberry-pi: use linux-firmware-brcm

Created by: ollieparanoid

Alpine's firmware packages have been updated to include the latest raspberry pi firmware as noted by @yangxuan8282 in https://github.com/postmarketOS/pmbootstrap/pull/1568#issuecomment-398620666. This commit adjusts our aports.

Details:

• remove obsolete aports/temp/linux-firmware
• firmware/firmware-pi-bluetooth: rename to device/bluetooth-raspberry-pi and remove firmware files (they are part of Alpine's linux-firmware now)
• device-raspberry-pi: depend on bluetooth-raspberry-pi
• device-raspberry-pi: not that Alpine's kernels depend non free firmware

@drebrez, @MartijnBraam, @PureTryOut, @yangxuan8282: Could you please review and test this?

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[x] Merge on GitHub (see https://postmarketos.org/merge)